Heat olive oil in a pot over medium heat.
Saute onion, garlic, and celery with a 1/2 teaspoon of salt for 3-5 minutes, stirring occasionally.
Add in cumin, coriander, red pepper flakes, and a few cracks of pepper and cook for 60 seconds more.
Add canned tomatoes, vegetable stock, sliced lemon and lentils to the pot. If you are using unsalted stock, add a teaspoon of salt at this time. Stir well.
Cover the pot and bring soup to a boil.
Reduce to a simmer and cook for 20-25 minutes until lentils are cooked and the flavors have deepened.
Taste for seasoning and add salt if needed.
If you'd prefer not to eat the lemon slices with their rind (I always do), remove them now.
Serve ladled into shallow bowls, sprinkled with fresh parsley.